Membrane hyperpolarization inhibits agonist‐induced synthesis of inositol 1,4,5‐trisphosphate in rabbit mesenteric artery.

1. Effects of membrane hyperpolarization induced by pinacidil on Ca2+ mobilization induced by noradrenaline (NA) were investigated by measuring intracellular Ca2+ concentration ([Ca2+]i), isometric tension, membrane potential and production of inositol 1,4,5‐trisphosphate (IP3) in smooth muscle cells of the rabbit mesenteric artery. 2. Pinacidil (0.1‐10 microM) concentration dependently hyperpolarized the smooth muscle membrane with a reduction in membrane resistance. Glibenclamide (1 microM) blocked the membrane hyperpolarization induced by 1 microM‐pinacidil. NA (10 microM) depolarized the smooth muscle membrane with associated oscillations. Pinacidil (1 microM) inhibited this response and glibenclamide (1 microM) prevented the action of pinacidil on both the NA‐induced events. 3. In thin smooth muscle strips, 10 microM‐NA produced a large phasic and a subsequent small tonic increase in [Ca2+]i with associated oscillations. These changes in [Ca2+]i seemed to be coincident with phasic, tonic and oscillatory contractions, respectively. Pinacidil (0.1‐1 microM) inhibited the increases in [Ca2+]i and in tension induced by NA, but not by 128 mM‐K+. Glibenclamide inhibited these actions of pinacidil. Pinacidil (1 microM) also inhibited the contraction induced by 10 microM‐NA in strips treated with A23187 (which functionally removes cellular Ca2+ storage sites), suggesting that membrane hyperpolarization inhibits Ca2+ influxes activated by NA. 4. In Ca2(+)‐free solution containing 2 mM‐EGTA, NA (10 microM) transiently increased [Ca2+]i, tension and synthesis of IP3. Pinacidil (over 0.1 microM) inhibited the increases in [Ca2+]i, tension and synthesis of IP3 induced by 10 microM‐NA in Ca2(+)‐free solution containing 5.9 mM‐K+, but not in a similar solution containing 40 or 128 mM‐K+. Glibenclamide (1 microM) inhibited these actions of pinacidil. These inhibitory actions of pinacidil were still observed in solutions containing low Na+ or low Cl‐. These results suggest that pinacidil inhibits NA‐induced Ca2+ release from storage sites through an inhibition of IP3 synthesis resulting from its membrane hyperpolarizing action. 5. In beta‐escin‐treated skinned strips, NA (10 microM) or IP3 (20 microM) increased Ca2+ in Ca2(+)‐free solution containing 50 microM‐EGTA and 3 microM‐guanosine triphosphate (GTP) after brief application of 0.3 microM‐Ca2+, suggesting Ca2+ is released from intracellular storage sites. Heparin (500 micrograms/ml, an inhibitor of the IP3 receptor), but not pinacidil (1 microM) or glibenclamide (1 microM), inhibited the Ca2+ release from storage sites induced by NA or IP3. These results suggest that membrane hyperpolarization is essential for the inhibitory action of pinacidil on the NA‐induced Ca2(+)‐releasing mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)